1. Field of the Invention
This invention relates to devices for indicating when a package of frozen food stored in a freezing chest may have thawed for a sufficient period of time for the food to deteriorate.
2. Prior Art
The process of quick freezing as applied to food and other products enables the food, when maintained in a frozen condition, to be preserved in its original fresh condition for an indefinite period of time. Frozen food, if thawed, can be re-frozen and maintained in a palatable and non-dangerous condition provided that it has not remained in its thawed condition for an extended period of time.
It is considered that to re-freeze food that has been thawed out is a dangerous practise since bacteria can propagate at a prodigious rate unless maintained in a frozen condition.
In some instances, freezer chests have been fitted with recording thermometers, however, this only indicates that maximum temperature has been reached, but gives no indication whether the food has reached a condition of complete thaw before power was re-established to re-freeze the food. Even if the duration of power failure is known, it will not establish whether the contents of the freezing compartment have thawed out, due to variences in the insulative characteristics of the equipment.
Some thaw indicators have been developed which indicate whether thawing has taken place but they do not distinguish whether thawing was superficial or complete. They, therefore, do not indicate any quantitative measure of whether the food merely started to thaw or did, in fact, thaw out completely. Therefore, it can not be known whether the food has been subjected to the dangerous condition of being completely thawed and then re-frozen.
These devices depend on the expansion of the ice to break a frangible partition, so that, if the temperature of the freezer rises above the melting point of the liquid, even for a short period of time, liquid from one compartment flows into the other compartment where it reacts with indicators to cause a calour change which is irreversible.
There are several major objections to devices of this type. They are very difficult and expensive to manufacture. Being of a fragile nature, they can be accidentally broken to give false readings. Once frozen, they can not be thawed and re-used. Consequently, they must be discarded whenever it becomes necessary to de-frost the refrigerator or if they have been activated on one occasion. They do not give any quantitative indication as to whether a superficial thawing or a complete thaw has taken place. Many indicators do not provide permanent color retention and many show little or no color if maintained in a frozen condition for long periods of time. Examples of such failures are evident in the use of phenolphthalein or methyl orange.